Technical Field
Embodiments of the subject matter disclosed herein generally relate to marine seismic acquisition systems, devices and methods, and more specifically to control of such systems for maintenance purposes.
Discussion of the Background
Seismic waves generated artificially for the imaging of geological layers have been used for many years. Reflection seismology is a method of geophysical exploration used to determine the properties of a portion of a subsurface layer in the earth, which information is especially helpful in the oil and gas industry. Marine-based seismic data acquisition and processing techniques are used to generate a profile (image) of a geophysical structure (subsurface) of the strata underlying the seafloor. While this profile does not necessarily precisely pinpoint the exact location of oil or gas reservoirs, it may suggest, to those trained in the field, the presence or absence of oil or gas reservoirs.
The overall seismic acquisition process includes generating seismic waves (i.e., sound waves) directed toward the subsurface area, gathering data associated with reflections of the generated seismic waves at interfaces between the layers of the subsurface, and analyzing the data to generate a profile (image) of the geophysical structure, i.e., the layers of the investigated subsurface. This type of seismic acquisition or exploration can be used both on the subsurface of land areas and for exploring the subsurface of the ocean floor.
Marine reflection seismology is based on the use of a controlled source that sends energy waves into the earth by first generating the energy waves in or on the ocean. By measuring the time it takes for the reflections to come back to one or more receivers (usually quite a few reflections, perhaps on the order of several dozen, or even hundreds), it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
One of the ways to perform marine seismic acquisitions or surveys is to tow an array of acoustic sources and receivers, some or both of which may be disposed on elongated streamers, by a vessel over the geographical area of interest (GAI) and to generate source signals and receive corresponding reflections while traversing the GAI. This process is sometimes referred to by those skilled in the art as “shooting” a GAI or cell being surveyed.
As marine seismic surveying has increased in sophistication, it has become possible to tow more sources and receivers behind a single vessel. Streamers can now be up to 16 km long, towed at approximately 5 kt, 10 m below the ocean surface. As streamers are generally neutrally buoyant, it is desirable to actively correct deviations of the streamers at the proper depth or course. The streamer therefore has “birds” arranged at intervals thereon. The birds are controlled from a dedicated system, by a streamer operator generally located in the tow vessel. The birds include “wings” (or fins or other control surfaces) movable to move the streamer both vertically (diving or surfacing) and horizontally (“lateral steering”). The birds and are other equipment attached to the streamer are generally not negatively buoyant, so it is desirable to maintain speed even if an element of a streamer, e.g., a seismic receiver, needs to be repaired so that the entire streamer floats. As used herein, the term “maintenance” encompasses repairs, cleaning, or other work performed by maintenance personnel or robots to restore a non-functional or less-than-fully-functional streamer to full functionality, or to forestall loss of functions of a streamer.
In order to maintain (e.g., repair or clean) the streamer, the birds in a portion of the streamer needing repair are controlled to surface that portion of the streamer while the remainder of the streamer stays submerged. A workboat is then dispatched from the towing vessel or a support vessel to make repairs on the surfaced section. The workboat generally includes a mounting fixture that grips a portion of the streamer while repairs are being made. However, the birds or the streamer itself can pose hazards to the workboat or workboat personnel. For example, if the birds are commanded to dive while the workboat is gripping the streamer, the workboat can be tipped or workboat personnel can be pulled or knocked overboard. Moreover, if a bird or streamer section adjacent to a bird is under repair, motion of the bird wings while it is being repaired can injure personnel or damage the workboat.
Prior schemes for mitigating these hazards require the streamer operator to transmit a “lock” command to the relevant birds to prevent motion of their wings for the duration of the work. However, this relies on the streamer operator to correctly command the birds, and requires that effective communications be possible between the workboat personnel and the streamer operator. Accordingly, there is a continuing need for ways of mitigating bird-related hazards during streamer maintenance.